The semiconductor industry is the foundation of the trillion dollar electronics industry. The semiconductor industry continues to meet the demands of Moore's law, whereby integrated circuit density doubles every 18 months, in large part because of continuous improvement of optical lithography's ability to print smaller features on silicon. This in turn depends in part upon identifying materials which exhibit sufficient transparency for practical use at ever-shorter wavelengths. For example, in photolithography, a circuit pattern is represented in a photomask, and an optical stepper is used to project the mask pattern onto a photoresist layer on a silicon wafer. Currently commercial scale photolithography is done at 248 nm. Lithography at 193 nm light is just entering early production. Current developmental efforts are directed to photolithography at 157 nm. A general discussion of photolithographic methods in electronics and related applications may be found in L. F. Thompson, C. G. Wilson, and M. J. Bowden, editors, Introduction to Microlithography, Second Edition, American Chemical Society, Washington, D.C. 1994
Polymers play a critical role in lithography in multiple area: one is the polymer pellicle which is placed over the mask pattern to keep any particulate contaminants out of the photomask object plane, thereby ensuring that the lithographic imaging will be defect free. The pellicle is a free standing polymer membrane, typically 0.8 micrometers in thickness, which is mounted on a typically 5 inch square frame. The pellicle film must have high transparency or transmission of light at the lithographic wavelength for efficient image formation and must neither darken nor burst with prolonged illumination in the optical stepper. Typical commercial processes utilize pellicles with >99% transmission through exploitation of polymers with very low optical absorption combined with thin film interference effects. The electronic industry requires greater than 98% transparency over an exposure lifetime of 75 million laser pulses of 0.1 mJ/cm2, or a radiation dose of 7.5 kJ/cm2.
A pellicle transmission of 98% corresponds to an absorbance A of approximately 0.01 per micrometer of film thickness. The absorbance is defined in Equation 1, where the Absorbance A in units of inverse micrometers (μm−1) is defined as the base 10 logarithm of the ratio of the substrate transmission, Tsubstrate, divided by the transmission of the sample, consisting of the polymer film sample on the substrate, Tsample, divided by the polymer film thickness, t, in micrometers.
                                          A            film                    ⁡                      (                          µm                              -                1                                      )                          =                              A            /            um                    =                                                                      Log                  10                                ⁢                                  ⌊                                                            T                      substrate                                        /                                          T                      sample                                                        ⌋                                                            t                film                                      .                                              Equation        ⁢                                  ⁢        1            
Certain perfluoropolymers have been identified in the art as useful for optical applications such as light guides, anti-reflective coatings and layers, pellicles, and glues mostly at wavelengths above 200 nm
WO 9836324, Aug. 20, 1998, Mitsui Chemical Inc., discloses the use of perfluorinated polymers, optionally in combination with silicone polymers having siloxane backbones, as pellicle membranes having an absorbance/micrometer of 0.1 to 1.0 at UV wavelengths from 140 to 200 nm.
WO 9822851, May 28, 1998, Mitsui Chemicals, Inc., claims the use at 248 nm of low molecular weight photodegradation-resistant, polymeric adhesives consisting largely of —(CF2-CXR) copolymers in which X is halogen and R is —Cl or —CF3. Higher molecular weight polymers such as poly(perfluorobutenyl vinyl ether), poly[(tetrafluoroethylene/perfluoro-(2,2-dimethyl-1,3-dioxole)],
poly(tetrafluoroethylene/hexafluoropropylene/vinylidene fluoride),
poly(hexafluoropropylene/vinylidene fluoride), or poly(chlorotolyl
fluorethylene/vinylidene fluoride) are disclosed as minor components to improve creep resistance. Only poly(chlorotrifluoroethylene) was exemplified.
Japanese Patent 07295207, Nov. 10, 1995, Shinetsu Chem. Ind Co, claims double layer pellicles combining Cytop CTXS (poly(CF2═CFOCF2CF2CF═CF2)) with Teflon® AF 1600 for greater strength.
U.S. Pat. No. 5,286,567, Feb.. 15, 1994, Shin-Etsu Chemical Co., Ltd., claims the use of copolymers of tetrafluoroethylene and five membered cyclic perfluoroether monomers as pellicles once they have been made hydrophilic, and therefore antistatic, by plasma treatment.
European Patent 416528, Mar. 13, 1991, DuPont, claims amorphous fluoropolymers having a refractive index of 1.24-1.41 as pellicles at wavelengths of 190-820 nm. Copolymers of perfluoro(2,2-dimethyl-1,3-dioxole) with tetrafluoroethylene, chlorotrifluoroethylene, vinylidene fluoride, hexafluoropropylene, trifluoroethylene, vinyl fluoride, (perfluoroalkyl)ethylenes, and perfluoro(alkyl vinyl ethers) are cited.
Japanese Patent 01241557, Bando Chemical Industries, Ltd., Sep. 26, 1989, claims pellicles usable at 280-360 nm using (co)polymers of vinylidene fluoride (VF2),
tetrafluoroethylene/hexafluoropropylene (TFE/HFP),
ethylene/tetrafluoroethylene (E/TFE), TFE/CF2═CFORf,
TFE/HFP/CF2═CFORf, chlorotrifluoroethylene (CTFE), E/CTFE, CTFE,VF2 and vinyl fluoride (VF).
Japanese Patent 59048766, Mar. 21, 1984, Mitsui Toatsu Chemicals, Inc., claims the use of a stretched film of poly(vinylidene fluoride) as having good transparency from 200 to 400 nm.
French et al, WO0137044, discloses vacuum ultraviolet (VUV) transparent materials exhibiting an absorbance/micron (A/micrometer) ≦1 at wavelengths from 140-186 nm comprising amorphous vinyl homopolymers of perfluoro-2,2-dimethyl-1,3-dioxole or CX2═CY2, where X is —F or —CF3 and Y is H, or amorphous vinyl copolymers of perfluoro-2,2-dimethyl-1,3-dioxole and CX2═CY2.
French et al, WO0137043 discloses ultraviolet transparent materials exhibiting an absorbance/micron (A/micrometer) ≦1 at wavelength from 187-260 nm comprising amorphous vinyl copolymers of CX2═CY2, wherein X is —F or —CF3 and Y is H and 0 to 25 mole % of one or more monomers CRaRb═CRcRd in the case where the CRaRb═CRcRd enters the copolymer in approximately alternating fashion, or 40 to 60 mole % of one or more monomers CRaRb═CrcRd in the case where the CRaRb═CrcRd enters the copolymer in approximately alternating fashion where each of Ra, Rb, and Rc is selected independently from H or F and where Rd is selected from the group consisting of —F, —CF3, —ORf where Rf is CnF2n+1 with n=1 to 3, —OH (when Rc═H), and Cl (when Ra, Rb, and Rc═F).
Japanese Patent Application Kokai Number P2000-305255A Shin-Etsu Chemical Company discloses copolymers containing >70% perfluorodimethyldioxole and 0-30 mole % tetrafluoroethylene, trifluoroethylene, difluoroethylene, vinylidene fluoride, and hexafluoropropylene for use as pellicles at 158 nm.
Japanese Patent Publication P2000-338650AShin-Etsu Chemical Company discloses copolymers containing >20% of perfluoroalkoxy substituted dioxoles such as 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole with F-containing radically polymerizing monomers such as tetrafluoroethylene, trifluoroethylene, difluoroethylene, vinylidene fluoride, and hexafluoropropylene for use as pellicles at 157 nm.
U.S. patent publication 20010024701 from Asahi Glass Company discloses fluorine containing polymers having a polymer chain consisting of carbon atoms wherein some chain carbons are substituted with fluorine and unspecified fluorine-containing groups. Encompassed in the disclosure are numerous polymers which are unsuitable in practice for use in applications at 157 nm because they are strongly absorbing or highly crystalline with concomitant high light scattering. Pellicles are inoperable without reasonably high transparency and yet the claims as written could include 100% opaque materials and fails to teach any method by which highly useful and completely useless polymer candidates for such applications can be distinguished from one another.
Many of the fluoropolymers cited in the references above are noticeably hazy to the eye because of crystallinity and are therefore unsuitable for applications requiring high light transmission and the projection of precision circuit patterns. Poly(vinylidene fluoride), poly(chlorotrifluoroethylene), poly(tetrafluoroethylene/ethylene), commercially available poly(tetrafluoroethylene/hexafluoropropylene) compositions, and poly(ethylene/chlorotrifluoroethylene) are all such crystalline, optically hazy materials. More recent references have thus been directed at amorphous perfluoropolymers such as Cytop® and Teflon® AF because they combine outstanding optical clarity down to at least 193 nm, solubility, and a complete lack of crystallinity.
Absorption maxima for selected hydrocarbon and fluorocarbon compounds are shown in Table 1. For hydrocarbons H(CH2)nH the data for n=1-8 is cited in B. A. Lombos et al Chem. Phys. Lett., 1967, 42. For fluorocarbons F(CF2)nF the n=3-6 data is cited in G. Belanger et. al., Chem. Phys. Letters, 3, 649(1969) while the datum for n=172 is cited in K. Sekl et al, Phys. Scripta, 41, 167(1990).
TABLE 1Comparison of UV Absorption Maximafor Hydrocarbons and FluorocarbonsWAVELENGTH OFABSORPTION MAXIMUMCnH2n+2CnF2n+2n = 1143 nm & 128 nmn = 2158 nm & 132 nmn = 3159 nm & 140 nm119 nmn = 4160 nm & 141 nm126 nmn = 5161 nm & 142 nm135 nmn = 6162 nm & 143 nm142 nmn = 7163 nm & 143 nmn = 8163 nm & 142 nmn = 172161 nm
As can be seen from the table, UV absorption maxima move to longer wavelengths as chain length increases for both hydrocarbons and fluorocarbons. Perfluorocarbon chains (CF2)n absorb at 157 nm somewhere between n=6 (142 nm) and n=172 (161 nm) while hydrocarbon chains (CH2)n absorb at 157 nm perhaps as early as n=2. But, as long as chain lengths offering acceptable transparency are limited to (CH2) or (CH2)6, perfectly transparent polymers at 157 nm and somewhat longer wavelengths would seem precluded according to the known art. Consistent with this, V. N. Vasilets, et al., J. Poly. Sci, Part A, Poly. Chem., 36, 2215(1998) for example report that various compositions of poly(tetrafluoroethylene/hexafluoropropylene) show strong absorption and photochemical degradation at 147 nm. Similarly the inventors hereof have found that 1:1 poly(hexafluoropropylene:tetrafluoroethylene) is highly absorbing at 157 nm
The absorbance per micron of a polymer will determine the average transmission of an unsupported pellicle film made from that polymer. For any particular polymer, the pellicle transmission can be increased, through the use of a thinner pellicle film thickness. This approach to increasing the pellicle transmission has a limited range of utility, since the pellicle film must have sufficient mechanical strength and integrity. These mechanical requirements suggest the use of polymer with relatively high glass transition temperature Tg and polymer film thicknesses of 0.6 microns or greater.